The present invention is related generally to the switching and demodulating of complex signals which include plural independent subchannels and in particular to a system and method of using an IF buss to simultaneously provide a complex RF signal for demodulation by a set of plural demodulators.
As the capability of processing RF signals which carry data has increased, so has the complexity and the amount of information carried by such signals. Often present communications systems utilize an RF signal which includes plural IF signals appropriately combined together (as, for example, by frequency modulation) and in which each of the IF signals includes plural (perhaps thousands) of independent baseband signals (such as encoded voice signals) which are themselves appropriately combined together (as, for example, by frequency modulation). For example, in a proposed communications system, as illustrated in FIG. 1, a beam signal 10 may include plural data or circuit signals 16, each of the data signals 16 containing voice data (or other data suitable for digital transmission). By known techniques, each of the data signals 16 may be modulated by independent subcarriers which are selected to be spaced sufficiently apart from other subcarriers so that the modulated data signals do not overlap (or interfere) with each other. Plural of such beam signals 10 may, in turn, be modulated onto an RF signal 14 by, for example, frequency multiplexing each of the beam signals to separate portions of the signal spectrum of the RF signal 14. Circuit guard bands 12 and/or beam guard bands 18 may be spaced between adjacent beam signals 10 and/or data signals 16, as is well known in the art. The size of the guard bands may be as needed for the particular signals being used.
As shown in FIG. 2, the invention may be used in a satellite communication system where plural circuit (data) signals 16 (e.g. telephone conversations) are transmitted to a satellite 20 where they are frequency spread modulated onto a beam signal 10 as shown in FIG. 1. Plural such beam signals 10 are frequency spread modulated onto a RF signal 14, as shown in FIG. 1, which in turn is transmitted to a ground station 30 on Earth. The ground station 30 downconverts the RF signal 14 into an IF signal 15 for transmission over a medium such as coaxial cable or any other suitable medium. The ground station 30 must then demodulate the IF signal 15 into its individual circuit signals 16 for integration into a terrestrial telephone network. It is noted that the ground station 30 also receives circuit signals 16, modulates them onto an RF signal 14, and transmits the RF signal 14 to a satellite 20. Similarly, the satellites receive RF signals 14 and demodulate them into individual circuit signals 16.
Prior art satellite communication systems also used RF signals 14 with plural beam signals 10 frequency spread modulated thereonto, each beam signal 10 in turn having plural circuit signals 16 frequency spread modulated thereonto. As shown in FIG. 3 and in contrast to the present invention, the prior art satellite systems often used switching techniques and fixed frequency demodulators to isolate individual circuit signals 16. In a satellite communication system, a RF signal 14 is usually capable of carrying thousands of circuit signals 16 and a ground station may receive several RF signals 14 simultaneously for processing. The switching required in the prior art system to route a signal to the proper fixed frequency demodulator is very complex. For example, as shown in FIG. 3, an RF signal 14 received at a ground station may first be downconverted to an IF signal 15 by a conventional downconverter 40. In turn, the IF signal may be applied to a demultiplexor 42 to extract each of the beam signals 10 from within. Each of the beam signals 10 may then be switched by a switch 44 to associate the proper beam signal with its related set of demodulators. Then, the beam signals may be demodulated into individual circuit signals 16 by a set of fixed frequency demodulators 46. Note that any particular demodulator in the prior art system generally receives only a particular beam signal 10 and is fixedly tuned to the carrier frequency of a particular circuit signal. As a result, prior art systems generally require a demodulator for each potential circuit regardless of how many demodulators are actually used. An additional result of the configuration of the prior art system is a limitation of the ability of the communication system to place specific circuit signals within the beam signals, as only the circuit signals associated with the set of particular demodulator frequencies associated with a beam signal may use a particular beam signal.
In the present invention, the entire IF signal 15 may be provided to each demodulator as opposed to the prior art where only a particular beam signal 10 is provided to each demodulator. Each demodulator, in the present invention may be frequency agile, that is, each demodulator may be variably tuned so that it can demodulate any circuit signal 16 from any beam signal 10. The present invention thereby avoids the complicated switching of the prior art, and it also allows for use of fewer demodulators than what would be necessary to handle the maximum traffic of circuit signals 16. This in turn would allow for statistically sizing the number of tuning demodulators in any of several manners known to those skilled in these arts.
For example, if the maximum physical capacity of a system was for 5,000 modulated circuit signals 16 but empirical data shows that only 2,500 circuit signals 16 are received and demodulated at any one time, the present invention could be constructed with only 2,500 frequency agile or tuning demodulators. The prior art system would still require 5,000 demodulators since each particular demodulator is dedicated to a particular beam frequency and a particular circuit frequency. The flexibility of the present invention maximizes resources by allowing for the use of only as many tuning demodulators as are needed.
While the demodulators of the present invention do not need to be frequency agile, some of the benefits of the present invention are lessened without such frequency agile demodulators. For instance, each of the demodulators could be fixedly tuned to a particular beam signal carrier frequency and a particular circuit signal carrier frequency. While this arrangement would have the advantage of obviating the complicated switching of the prior art, it would still require the number of demodulators to be equal to the maximum number of circuit signals 16 able to be modulated onto the RF signal 14.
It is accordingly an object of the invention to obviate these and other disadvantages of the prior art and to provide a novel system and method for isolating in baseband form a modulated circuit signal 16 from a beam signal 10 modulated on an IF signal 15.
It is another object of the invention to provide a novel system for isolating in baseband form a modulated circuit signal 16 from a beam signal 10 modulated on any of plural IF signals 15.
It is a further object of the invention to provide a novel satellite communication system with means for isolating in baseband form a modulated circuit signal 16 from a beam signal 10 modulated on any of plural IF signals 15.
These and many other objects and advantages of the present invention will be apparent to one skilled in the art from the claims and from the following detailed description when read in conjunction with the appended drawings.